'Jump Out Boys' is a phrase used to describe police, detectives, undercover officers, and other law enforcement personel.
More specifically the term refers to officers involved in sting opereations. (Narcotics, Prostitution, Organized Crime etc.)
The 'Jump Out Boys' are usually either the swat team, or the cops on stake outs inside the sound van, listening in on wire taps or wired informants. When tipped off, or alerted, these officers are known to jump out of the vehicle and subdue suspects by force, thus the term 'Jump Out Boys'.
They ride in unmarked vehicles, typically vans, trucks, and vehicles without windows.
Some gangs and/or squads are known tocall themselves 'Jump Out Boys' due to their simmilar, jump out style.

"See that U-Haul across the street, It's the 'jump out boys'. They been watchin us for three days."
"I got hemmed up by the 'jump out boys' last night. I was walkin home from the spot and they came out of nowhere!"

Unmarked police rolling down the Blade, four or more of them in each single large tint-windowed SUV or van with no back windows, waiting for a vice officer to confirm having positively identified a target — a prostitute, pimp, dealer, or someone with an outstanding warrant or who was banned by the courts from the area for previous prostitution or drug activity — or, after seeing such activity taking place or identifying a known suspect themselves as they go by, they roll up to the target(s), quickly stop and all jump out to arrest/search/question or otherwise harass everyone involved.

Unmarked police rolling down the Blade, four or more of them in each single large tint-windowed SUV or van with no back windows, waiting for a vice officer to confirm having positively identified a target — a prostitute, pimp, dealer, or someone with an outstanding warrant or who was banned by the courts from the area for previous prostitution or drug activity — or, after seeing such activity taking place or identifying a known suspect themselves as they go by, they roll up to the target(s), quickly stop and all jump out to arrest/search/question or otherwise harass everyone involved.